EPITOMEOFELECTRICITY.
If a glass tube be rubbed in the dark with a dry hand or piece of buckskin, upon applying the knuckle to it a luminous stream or spark will appear, passing from the glass to the knuckle, attended with a crackling noise: this luminous spark or stream is called electricity.[8]It is produced by the friction of several other substances, and was first observed on amber.—Hence its name, fromηλεκτρονthe greek term for amber. It is a fluid extremely subtle, abounding in all nature, and is one of her principal agents; which, though generally imperceptible, sometimes becomes the object of our sight and other senses.
A glass tube, having been rubbed and producing the appearances above described, is said to beexcited. Thehand or buckskin, by which this is effected, is called therubber.Electricsare all substances which can be made to produce the same appearances; the most perfect are glass, amber, sealing-wax, sulphur, gum lac, rosin &c. These are also callednon-conductors, from their inability to conduct the electric fluid.Conductorsornon-electricsare those bodies which cannot be excited, but have the power of transmitting electricity; such are metals, water, the bodies of animals, an imperfect vacuum, heat &c. But strictly speaking, there are noperfectconductors or non-conductors.
A body is said to be in itsnatural state, when it is in the same state, with respect to electricity, as the mass of the earth.
When a body has more of the electric fluid than its natural quantity, it is said to beelectrified positively, when less,negatively; but neither of these cases can occur in a conductor, unless the communication between it and the earth be cut off by theintervention of an electric or non-conductor. When this happens, the conductor is said to beinsulated.
It may not be amiss here to mention, that the termselectricor anelectric per se, andnon-electric, were at first made use of from an erroneous idea that only those called electrics, contained the electric matterin their substance, which was capable of being excited by friction, and communicated by them to those called non-electrics, and supposed to be destitute of it: for glass and other electrics, being rubbed, discovered signs of having it, by snapping on the approach of a finger or other conductor, and by attracting and repelling light bodies; while other substances could not be made to produce any such effect. It has however since beenproved by experiments, thatbothelectrics and non-electrics contain this matter in their substance; but that non-electrics cannot be excited, owing to the fluid diffusing itself through them as soon as collected. These terms are therefore improper, and as the only difference is that some bodies will conduct electricity and others will not, the termsnon-conductorandconductorare those which might generally be used with the most propriety in speaking on this subject; though, in conformity with custom, we shall often usenon-conductorandelectricas synonymous.
Those substances by which electrical phenomena may be produced, form the subject which next demands our attention; but these are so numerous that it would be vain to attempt to specify them all. Perhaps it may be doubted, whether every material substance, with the exception only of metals, water, and charcoal, may not be considered as an electric.
Some however exhibit particular phenomena more obviously than others; and hence a number of catalogues have been formed, for shewing the effects which arise when electrics are excited with different rubbers. The specification which we esteem the most complete, was formed by the ingenious Mr. Cavallo, and we shall give it in his own words.
“In the following table (says he) may be seen what electricity will be excited in different bodies, when rubbedwith different substances. Smooth glass, for instance, will be found by this table to acquire a positive electricity, when rubbed with any substance hitherto tried, except the back of a cat: (by which I mean the skin of a cat while on the animal alive:) rough glass, (viz. glass, the polish of which has been destroyed by emery or otherwise) will be found to acquire the positive electricity, when rubbed with dry oiled silk, sulphur &c. and the negative when rubbed with woolen cloth, the hand &c. and so of the rest.”
From the above table it appears, that the powers of electric substances vary prodigiously from one another; and that, according to the different rubbers made use of, we may sometimes produce one phenomenon and sometimesanother. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit; which may be done in the following manner.
First. Those which exhibit astrongandpermanentattractive and repulsive power, of which the most remarkable is silk.
Second. Those which exhibit the electric light, attraction, repulsion, and all the other phenomena of electricity in a veryvigorous, thoughnot durablemanner; of these glass is eminently preferable to all others.
Third. Those which exhibit electric appearancesfor any length of time, and which communicate to conducting bodies,the greatest electric power.—Of these, the substances callednegative electrics, such as sealing-wax, resinous substances, and resinous compounds, are the best.
Fourth. Those which readily exhibit electrical phenomena byheatingandcooling.—Of these, the tourmaline[9]is the principal.
The best method of disturbing the electric fluid, that is of making it pass from one body to another, is friction. This may be done either by rubbing one electric with another, or with a conductor; but the electricity is generally stronger in the latter case. Other methods for causing electrics to shew electric appearances, are,melting, or pouring a melted electric on another substance, heating and cooling, evaporating or effervescing.
All bodies in nature are, with reference to this subject, divided into two classes,electricsandconductors.
It has been fully demonstrated by experiment, that no substance which is a conductor can be excited so as to exhibit electrical phenomena: and in the same manner it has been found, that no substance which can be excited, is a conductor. But as we have already hinted, there is, strictly speaking, no substance which is aperfectconductor or non-conductor; because, on the one hand, the electric fluid meets with some resistance in its passage through the best conductors; and on the other, it is in part transmitted through, or passes over the surface of, most if not all electrics.
The two following lists contain as complete an enumeration of electrics and conductors as the present state of knowledge, in regard to electricity, permits us to make.
The substances are disposed in the order of their perfection; that is, the best conductors and the best electrics are placed at the head of their respective lists, and those of an inferior kind follow, somewhat in the manner of a scale graduated downward. Perfect exactness however is not to be here expected, because the subject forbids it, and some of the specified articles are of classes of substances among which there may be a sensible difference.
Conductors or non-electrics.
Gold,
Silver,
Copper,
Platina,
Brass,
Iron,
Tin,
Mercury,
Lead,
Semi-metals.
Metallic ores.—Of which those are the best which contain the greatest number of metallic parts and are nearest to a reguline state.
Charcoal, either of animal or vegetable substances—
Animal fluids,
Acids,
Saline substances,
Hot water,
Cold water,
Salt water,
All other liquids except oils,
Red hot glass,
Melted rosin,
Flame and the effluvia of flaming bodies,[10]
Ice and snow—but not below the temperature of 13° Fahrenheit.
Earthy and stony substances, of which the hardest are the worst.
Glass filled with boiling water,
Vapour or steam of boiling water,
Smoke.
All compounds which contain the above substances in different proportions, are conductors in different degrees.
Glass and all vitrifications; even those of metals.
All gems, of which the most transparent are generally the best.
All resinous substances and resinous compounds,
Amber,
Sulphur,
Baked wood—if not suffered to imbibe moisture.
All bituminous substances,
Wax,
Silk,
Cotton.
All dry animal excrescences; as feathers, hair, wool, horn, &c.
Paper,
White sugar and sugar candy,
Atmospheric air and other gasses,
Oils,
Dry and complete metallic oxyds,
The ashes of animal and vegetable substances,
All hard stones; of which the hardest are the best,
Powders not metallic.
Ice at and below the temperature of 13° of Fahrenheit’s thermometer. According to Mr. Walsh’s and Mr. Morgan’s experiments, the Torricellian vacuum ought to be placed at the head of this list; but the singular nature of a vacuum, though a non-conductor, will hardly entitle it to the name of an electric.
CHAP. IV.Of the electrical machine.
Having now explained the terms made use of in the study of electricity, and noted some of the phenomena of different electric substances, and the difference between electrics and conductors; we shall proceed to describe theelectrical machinemade use of for shewing experiments, and for exhibiting other electric phenomena to the best advantage.
The principal parts of the machine are,the electric,the rubber, themoving engine, and theprime conductor. We shall take notice of each of these parts separately and then describe the whole machine together.
Formerly different kinds of electrics were used; at present smooth glass is preferred before all others, as most convenient, and because it will, by itself, answer the purposes of several others. For when the machine has an insulated rubber, which is easily prepared, the operator may produce positive or negative electricity[11]at pleasure, without changing the electric.
With respect to the forms of the glass, those commonly used are globes, cylinders and plates. The most convenient size for a globe is from ten to twelve inches in diameter. It should have two necks, centrally opposite, which must be cemented[12]to strong caps, in order to adapt them to a proper frame. Cylinders are also made with two necks. Their common size is from sixto seven inches in diameter, and from ten to twelve inches in length; the glass generally used is the best flint.
It has long been questioned whether a coating[13]of some electric substance, has any effect in increasing the power of an electric; but now it seems pretty well determined, that if it does not increase the power of a good one, it at least considerably improves a bad one.
The next thing to be considered isthe rubberwhich is to excite the electric. This, as it is now made, consists of a cushion of buckskin, stuffed with hair or flannel, and fastened to a piece of wood well rounded at the edges; to this is glued a flap of Persian black silk, which goes over nearly one half of the cylinder or globe. The rubber should be supported by a small iron or brass spring, placed inside of it, as is represented edgewise by R, figure 2, in the frontispiece. This acts in a much more uniform and parallel manner than if it were placed under the cylinder. It suits any inequalities that may be on the surface of the glass, and by means of a screw may be made to press against the cylinder as occasion requires. It should likewise be insulated in the most perfect manner by glass, or by baked wood well varnished. But when experiments are to be made which do not require or admit of insulation, a communication must be made between the rubber and the earth, by a chain or conductor.
To increase the effect of the rubber several substances have been used with success, particularly whiting and pulverised chalk. But the best of all is an amalgam of zinc and mercury.[14]This amalgam is to be used byfirst applying a moderate quantity to the cushion; and afterwards by spreading it on a separate piece of leather, and applying it occasionally to the under part of the cylinder while turning. In this method of using it, only a small quantity of amalgam is consumed, while the glass is very strongly excited; and by degrees the whole rubber contiguous to the cylinder is covered with amalgam, in the form of a concave cake. It is with such a rubber that the cylinder is most powerfully excited.
An ingenious friend has favoured us with the following explanation of the manner in which electrics are excited, which to us is more satisfactory than any other we have seen. “In order that electricity may be accumulated in greater quantity in one body than in the surrounding ones, it must be set in motion. This may be effected by therubbing of electrics; thejuxta-positionof non-electrics of different conducting powers; and by thechemical actionof many, if not all bodies on each other. The rubber will act on the first principle, and the more perfect the contact between it and the electric the greater will be the effect. The chalk, whiting, amalgam &c. while they will, if properly prepared, make the contact more perfect, will also be of service on the second principle; and the amalgam will besides be of use on the third. Mercury and zinc may be exposed separately to the air without any alteration; but when combined they readily unite with the oxygen of the atmosphere; especially when the surface of contact is frequently renewed, and the temperature increased by friction.
“The glass, acquiring a different state of electricity from the rubber, will, as each portion passes from under it, carry away and impart to the prime conductorthe excess which it has obtained; and this the more certainly if the dissipation of the electricity be prevented, or the accumulation increased, by a piece of silk connected with the rubber.—The chain making the communication between the rubber and the adjoining non-electrics will enable this process to go on; and perhaps may also assist on the second principle.”
With respect to the engine which is to give motion to the electric, it has been customary, simply to turn the globe or cylinder with a winch; but this will not produce the greatest power of which the glass is capable. To effect this it should be made to turn six or seven times in a second, which is more than can conveniently be done with the winch only; and therefore multiplying wheels are used with advantage.
The prime or first conductor is an insulated non-electric substance, furnished with a number of points on the end towards the electric, in order to collect the electricity from it. It is usually made cylindrical, but whatever be its form it should always be perfectly free from points or sharp edges, except the points toward the electric already mentioned; and if holes are made in it, which on many accounts are very convenient, they should be well rounded and perfectly smooth.—The larger this conductor is, if not disproportionate to the cylinder or globe, the stronger and more dense will be the electric spark, which will proceed from it when touched by a blunt conductor. There must however always be a certain proportion between the cylinder or globe and the prime conductor, for if the former be small and the latter large, the electricity will not be collected fast enough, to preserve an accumulation of it in the prime conductor, because a portion is always takenoff by the air, in proportion to the surface presented to it by the conductor.
We shall now give a short connected explanation of the whole machine, a draft of which is exhibited in the frontispiece. AB and CD are two pillars of baked wood well varnished, perpendicularly raised from the top of the table EFGH—these serve to support the cylinder I, by the axles of the caps KK; from one of these proceeds the long axle L, which passes through a hole in the pillar CD, having the pulley M, fixed on its square end. N is a multiplying wheel, around which the band or strap O passes, and likewise around the pulley M.—The wheel N should be made moveable with respect to the pulley M, to accommodate the stretching of the band, or else the pulley should have a number of grooves of different radii in its circumference.
The rubber R, is fastened to a pillar of glass, or baked wood P. The pressure of the rubber may be augmented at pleasure, by means of a sliding board and tightening screw.
The prime conductor is represented by Q. It is insulated by the glass pillars SS, which support it. T represents the points which collect the electricity from the cylinder.
Cylinders and globes made for electrical machines are not always to be procured. Their place however, may be very well supplied by the large show bottles of the apothecaries. When these are used, one of the caps, instead of being concave (to receive the neck of the cylinder) must be made convex—so as to fit the hollow in the bottom of the bottle.—It is to be fastened with the cement used in the other machine.
The most powerful electrical machine ever constructed, was at Teyler’s museum at Haarlem. It had, instead of the cylinder or globe as in the common machines, two circular plates of glass, which were made to turn upon the same horizontal axis. These plates were excited by eight rubbers, which acted on their surfaces. In this machine the prime conductor had branches which collected the electricity from between the plates.
It is not necessary however in this form of the machine to have two plates, the second being added only to increase the power. The plate must be firmly fastened by its centre to an axis—so as to turn vertically between two uprights of baked wood, as in the construction of the cylindric machines; but in this case the uprights must be so close together, as barely to leave room for a rubber on each side of the plate. The rubbers may be made of the same form with that in the cylindric machine—except that they must have a projection at the back, to fit a niche cut in the uprights which support the plate. The power of the machine will be increased by having four rubbers; two above and two below the axis of the plate. The prime conductor is placed opposite one of the ends of the axis, and is divided at the end towards the electric into two branches or arms, which extend horizontally to the circumference of the plate, each of which is furnished with points to collect the electricity.
As plates are not always to be procured, a good substitute may be found in a thick pane of glass or a piece of an old looking-glass. Mark with a diamond or file a circle on the glass, of the size you intend for your plate. Then putting the plate into warm water, after some time cutthe glass with a diamond in tangents. The more numerous the cuts, the nearer the plate will be to a circle. A hole may be made in the centre for the axis, by scratching with a diamond, and grinding with a rod of iron (held between the hands) and emery.
Having described the electrical machine, we are now to consider some of the phenomena attending its operation. When the prime conductor receives electricity from the cylinder, it is said to beelectrified by communication, and it then acts in every respect like the cylinder itself, except that the latter, when touched by a conductor communicating with the earth, gives a considerable number of sparks before it is discharged; whereas the conductor discharges itself by a single spark.
The cause of this difference is that the cylinder, being an electric, cannot convey the electricity of all its surface to that part, to which the conducting substance is applied; but the fluid accumulated in the whole conductor, passing easily through its substance, is transmitted at once to the point from which the discharge is made. Hence it appears that the electricity discharged from an electrified conductor is more powerful than that discharged from an electric—the conductor acquiring a large quantity of electricity from an electric, by receiving it gradually, spark after spark, and afterwards, when touched, discharging it all at once.
The velocity of electricity is almost beyond conception. It is, notwithstanding, in a small degree relativeto the quantity put in motion, and to the goodness of the conductor by which it is transmitted. A large quantity of electricity passes through a good conductor with such rapidity, that there is no perceptible difference in the time which it takes to go one foot, or one thousand feet. A small quantity however has been found to take a time barely perceptible, in passing through a long and imperfect conductor. Experiments relative to this point will be related hereafter.
If a piece of metal be presented to an over-charged prime conductor, the fluid passes with violence from the one to the other; anelectric spark, having the appearance of fire, is seen flashing between them, and a snapping noise, like the cracking of a whip, is heard. If this piece of metal be insulated, the prime conductor will be only partially discharged, that is, the redundant electricity will be divided between it and the piece of metal, nearly in proportion to their surfaces. This electric spark has not only the appearance of fire, but, when large, will actually set fire to a variety of easily inflammable substances; such as cotton sprinkled with rosin, spirits of wine &c. This power of exciting flame is not commonly believed to arise from any culinary heat in the electric spark, because if the spark be small it will not excite flame in substances the most inflammable. It acts probably by friction on the same principle as the rubbing of sticks against each other produces fire.
The electric spark, taken upon any part of a living animal, causes an unpleasant sensation, which is more or less pungent and disagreeable, as the spark is stronger or weaker, and the part more or less delicate.
There is a slight difference between the appearance of a spark taken from a body positively electrified, and that from one negatively electrified. The former, if not very long, appears straight and sharp; the latter is generally ramified, or appears in a zig-zag line.
The noise which attends the spark, is caused by the sudden agitation into which the air is thrown, by its passage through it.
If an uninsulated conductor, which is broad, round and polished at the end, be presented to the prime conductor, a short and dense spark, accompanied with some noise, will be perceived; if the conductor be less broad, the spark will be longer, less dense, and attended with less noise; if the breadth be still more diminished, so that the conductor may come under the denomination of a point, the electric matter will pass to it, from the prime conductor, and through a greater space, with a hissing noise, and in a continual stream; a still greater sharpness will enable the electricity to pass over a yet more extended space, but unaccompanied by noise, and only a small light will be seen upon the point. The same result will arise if points of different acuteness be affixed to the prime conductor, instead ofthe uninsulated one: but if both be pointed, the electricity will be more readily discharged.
In all the above cases, the appearance of the electric matter at the point, will indicate the kind of electricity from which it proceeds. A large divergent cone indicates positive electricity; a small globular light, that which is negative. Hence it is always easy to ascertain whether an insulated conductor be electrified positively or negatively, by presenting a point to it, as the light at the point is always definitive of the contrary electricity in the conductor.
If a pointed conductor be electrified, either positively or negatively, and the face be brought near the point during the electrization, a wind will be felt blowing from the point, accompanied with a peculiar sensation, commonly calledthe spider’s web. It is remarkable that the current of air is always in the same direction, whether the point throws off or receives electricity.
The re-action of the force, by which the air is put in motion, is exerted upon the pointed body. This is shewn by a very pleasing experiment called the electric fly. This fly is composed of four small wires, fastened into a metallic cap, similar to those used in sea-compasses, so that the wires may easily move upon a point, in a horizontal direction. They should be exactly balanced, and have their ends, which must be very sharp, all bent in the same direction. Now if this fly be placed on an insulated point and electrified, its sharp ends will become luminous in the dark, and it will revolve in a direction contrary to that in which the ends are bent; or if it be placed on an uninsulated point and brought near the electrified prime conductor, the same effect will follow.
It is to be observed, that the fly will move round in the same direction, whether electrified positively or negatively. The cause of this seeming contradiction depends upon the repulsive power existing between bodies possessed of thesameelectricity; for the air opposite to the points acquires a strong electricity, analogous to that of the points, it is therefore repelled, and replaced by other air, which is also electrified and repelled. Hence a continual stream is produced, blowing from the points, and that equally, whether the electrization be positive or negative; and as action and re-action are equal and in contrary directions, the points, repelling the air, must themselves be repelled, and in the opposite direction; which causes the fly to be always turned one way, that is, in a direction contrary to that in which the air is moved.
In vacuo no motion is produced, because there is no air on which the electric matter can act when it issues from the points.
In like manner, if air be confined in a receiver, the motion of the fly soon ceases, because the fluid cannot pass through the air and the glass. But on applying the end of a finger to the outside of the receiver, opposite one of the points of the fly, the motion will begin again, and by moving the finger occasionally round the glass, it may be continued till most of the glass is charged.
The cause of this motion is, that when the finger is applied to the outside of the receiver, the glass, loosing part of its natural quantity of electricity from that side, (i. e. when the fly is electrified positively, and vice versa if negatively) takes up the fluid from the air on its inner surface. Hence the air becomes capable ofbeing again electrified by the point and this renews the motion.
We have already stated that if a pointed wire be presented to a conductorpositivelycharged, it will be illuminated with a star or globe; and if the conductor be negatively charged, the illumination will have the form of a pencil or divergent cone. F. Beccaria explains this in the following manner. I suppose, says he, that the star is occasioned by the difficulty with which the electric fluid is extricated from the air, which is an electric; suppose for instance that a pointed wire is presented to a body positively electrified; the electric fluid is first communicated from that body, to the air between it and the pointed wire, and then the wire must extricate it from the air.
The pencil is occasioned by the force with which the fluid, issuing from the point, passes through the contiguous air to that which is more remote, i. e. by dividing the contiguous air, and not by affixing itself to it.
Beccaria likewise remarks, that if two equally sharp pointed bodies are brought near the prime conductor, they will appear luminous at only half the distance that one of them would. They will also discharge it in half the time.
It will not be improper to remark here, that when a point not electrified is opposed to one electrified positively, both points will have small globular lights upon them; but if a positive one be opposed to one negatively electrified, they both preserve their own characteristic properties.
From the above the following conclusions may be drawn.
First, That pointed bodies attract the electric matter more or less easily, and at a greater or less distance, according to their acuteness.
Second, That pointed bodies have the power of attracting electricity as well as of repelling it, in a greater degree than conductors of any other form.
We shall treat farther of pointed conductors under the articleThunder-house.
No satisfactory theory of electric attraction and repulsion has, so far as our knowledge extends, ever yet been given. The phenomena have been differently accounted for, as the writers have embraced different opinions in regard to positive and negative electricity. One mode of explanation has been adopted by those who believe, with Franklin, that positive electricity is only an accumulation of the electric fluid in a body beyond its natural state; and that negative electricity is nothing more than a deficiency of this fluid in a body. Another mode of explanation is given by those who maintain, in opposition to Franklin, that positive and negative electricity are either two distinct fluids, or else vibrations of the same fluid—the positive electricity always rushing out of a body, and the negative always rushing in. Those who maintain this hypothesis endeavour to support it by the easy solution which they affirm it gives to the phenomena of electric attraction and repulsion. But after a careful examination of this theory, we think that, so far from being satisfactory, itis scarcely intelligible. We therefore do not choose to introduce it into our epitome, as affording any solution of the difficulties that occur on this part of our subject. We are besides of opinion that the evidence in favour of a single fluid is conclusive, as we shall show when we come to discuss the theory of electricity. Yet we confess that we cannot, on this theory, offer a rationale of electric attraction and repulsion, that satisfies ourselves. It is therefore the demand of candour, and in the spirit of the Newtonian philosophy, to avow explicitly that this part of our subject is yet involved in much obscurity. In the mean time we are acquainted with certain facts, and with the clear explanation which they give of certain phenomena.
1. That bodies positively electrified, repel each other.
2. That bodies negatively electrified, also, repel each other.
3. That bodies positively electrified, attract those which are negatively electrified.
4. That bodies either positively or negatively electrified, induce a contrary electricity in bodies in their natural state, brought within the sphere of their action.
This statement is easily verified by experiment, in the following manner.—By flaxen or hempen threads, suspend, from the prime conductor, two balls made of cork or elder-pith, so that they touch each other. On charging the conductor, these balls, being both electrified positively, will immediately repel each other, and be separated to a considerable distance.—Remove one of the balls, take it in your fingers, and bring it near to the one which remains positively electrified, and the two will immediately rush together; because there are now two substances of which one is electrified positively,and the other negatively.—Again. Suspend two balls, of the kind just mentioned, from an insulated cushion of an electric machine, and let them touch each other. Put the machine in motion and the balls, which are now both electrified negatively, will repel each other and separate, as in the case first described.
In attempting to explain the first of these phenomena Dr. Franklin once supposed that there was an electric atmosphere round each of the balls positively electrified, the particles of which atmosphere, by mutually repelling each other, separated the balls. He also supposed that as bodies negatively electrified, or not having their proportional quantity of the electric fluid, are always strongly disposed to receive it, this would account for the fact that when one of these bodies was brought near to one that had more than its proportional quantity, the two would naturally rush together; the one to impart, and the other to receive the fluid. But at this time he was not acquainted with the fact, that two bodies negatively electrified would repel each other. When this was discovered he candidly acknowledged the utter deficiency of his theory, in regard to electric attraction and repulsion. Some of his friends and followers, however, have endeavoured still to maintain it. But we think that though their zeal has been greater, their success has not exceeded that of the Doctor himself: and we have already stated that other theories are equally, if not more defective, than that of Franklin. Let us then leave the explanation of electric attraction and repulsion to be made when future and fortunate discoveries shall have furnished the means of making it, and let us proceed with the application of known facts and principles.
A pleasing exhibition of the phenomena of electric attraction and repulsion, may be made in the following manner.
Take a glass tube, and after having rubbed it, let a small light feather fall from your fingers, at the distance of eight or nine inches from it.—The feather will be immediately attracted by the tube and stick very close to its surface for some seconds, after which it will be repelled, and if the tube be kept under it, the feather will continue floating in the air, at a considerable distance from the tube, without coming near it again, except it touch some conducting substance; and if you manage the tube dexterously, you may drive the feather through the air of the room at pleasure.
The cause of this phenomenon is obvious. The feather, at first, not being electrified, rushes to the excited tube. There it becomes electrified and is then repelled, and cannot approach the tube again, unless it first touch some conducting substance; because it cannot part with its electricity while floating in the air, and therefore cannot acquire a contrary electricity; consequently it must remain in a state incapable of being again attracted by the excited tube.
There is a remarkable circumstance attending this experiment, which is, that if the feather be kept at a distance from the tube by the force of electric repulsion it always presents the same part towards the tube. The reason of this phenomenon is, that the equilibrium of the fluid in the different parts of the feather being once disturbed cannot easily be restored; the feather being an electric, or at least a very bad conductor. When the feather has acquired a quantity of electricity from the tube it is plain that, by the action of the excitedtube, that superinduced electricity will, for the most part be forced to that side of the feather which, at first, happened to be farthest from the tube; hence that part will always afterwards be repelled the farthest.
This experiment may be agreeably varied in the following manner.—A person may hold an excited tube of glass, within a foot and a half of a stick of sealing-wax, or any other electric negatively electrified, held by another person; a feather let fall between these differently excited electrics will leap from one to the other alternately, and the two persons will seem to drive a shuttlecock by the force of electricity.
Another experiment calculated to shew the phenomena of electric attraction and repulsion is theelectric spider.
Cut a piece of cork in the shape of a spider, and run a few short threads through it, to represent the legs; this done, suspend it by a silk thread from the ceiling of the room, or any other support, so that the spider may hang mid-way between the knob of a jar and the knob of a wire fastened to the table, or to the outside coating of the jar when not charged; let the place where the jar stands be marked; then charge and replace it. The spider will now begin to move from knob to knob, and continue this motion for a considerable time.
In this case, the knob of the jar is charged positively, and the spider, being in its natural state, is attracted by it; the knob then communicates to it some of its electricity, and the spider becoming possessed of the same electricity with the knob, is repelled by it, and immediately runs to the other knob, which communicates with the negative coating, or with the table, where it discharges its electricity and is again attracted by theknob of the jar. This attraction and repulsion continue till the jar is discharged, when the spider finishes its motion and seemingly expires.
This consists of a glass phial, jar, or bottle, coated on the outside and inside with tin-foil, rendered adhesive by paste or gum water. About two inches of the glass at the top are left without any metallic covering, to prevent a communication between the outside and inside coatings, while the electricity is collecting.—The mouth of the phial or jar is furnished with a cork which receives a wire, ending in several ramifications which touch the inside coating. The upper end of this wire, which should extend a convenient distance above the mouth of the jar, is furnished with a metallic ball.
When the phial or jar is to be charged, it may be held in the hand or placed on an uninsulated table, with the knob of the wire touching the prime conductor. The inner surface of the glass now acquires the same electricity with the prime conductor, and the external one acquires a contrary electricity by means of its uninsulated coating.
When a phial similar to the one above described is highly charged, a spontaneous discharge will usually take place over the uncoated surface, and seldom through the glass. But if the uncoated surface be left larger than from two to three inches, the phial is more apt to crack and become useless, by the charge passing through the glass. There is not however an absolutecertainty that a jar which has once discharged itself over its surface will not, at another time, break by a discharge through the glass.
It was long disputed whether the discharge of the Leyden phial resided in the coating or in the electric. The following experiment clearly decides, that its residence is in the electric.
Upon an uninsulated plate of metal, lay a plate of glass considerably larger, so that there may be a rim of three or four inches projecting beyond the metal. Upon the glass lay another piece of metal, of the same size with the first, and so as precisely to cover it.
Let this instrument be charged, by connecting the upper metallic plate with the prime conductor. Then separate the metallic plates from the glass; and upon examination the glass will be found to possess the contrary electricities on its opposite sides; that side which during the electrization communicated with the prime conductor will have a like electricity with it, and the other the contrary.
Discharge the electricity of the metallic plates, and replace the whole apparatus in its former situation.—Take a discharging rod, formed by a piece of bent wire with a metallic ball at each end; touch the under plate and bring the other end of the wire near the upper plate. The consequence will be, that a strong and loud spark will pass between the upper plate and the discharging rod; the electricity of the glass will be discharged, and there will afterwards remain no signs of electricity, either in the glass, or the metallic plates.—Hence it appears that the electricity resides in the glass, and that the coatings, whether in a plane or spherical form, are of no other use than to convey the electricfluid to the glass; to keep it equably distributed over the surface; and to form a communication between the different parts of the electrified glass, so that the discharge from them may be simultaneous.
When the discharge of a coated electric is made through the body of a living animal, it occasions a sudden motion, by contracting the muscles through which it passes, and gives a disagreeable sensation commonly called theelectric shock.
When a greater degree of electric force is required than a single jar is capable of giving, the electrical battery is made use of as part of the apparatus, which takes its name from the formidable effects it produces. This battery consists of a number of coated jars, placed in such a manner that they may all be charged at the same time, and discharged in an instant; so that the whole force of electricity accumulated in them, may at once be exerted on the substance exposed to the shock.
In discharging electrical jars, the electricity goes in the greatest quantity through the best conductors, and by the shortest passage. Thus if a chain and a wire be made to communicate at the same time with the outer coating of a jar, and be both presented to the knob of that jar, the greater part of the charge will pass by the wire, and very little by the chain, because the latter is a worse conductor than the former, on account of its discontinuation at every link. When the discharge ismade by the chain only, sparks are seen at every link, which is a proof they are not in contact.
The force of an electric shock is not affected by the inflections of a conductor through which it passes, though it is sensibly weakened by its length. Hence, when the circuit or communication between the two sides of a Leyden phial is formed by one person applying his hands to the different sides, the shock is stronger than when it is formed by many persons joining hands. Yet a considerable shock was given by the Abbè Nollet, in the presence of the king of France, to one hundred and eighty men; who formed an electrical circuit.—They were all shocked in the same instant.
Doctor Watson and many other gentlemen of eminence in science, were at the pains of making experiments of the same kind. They found, by means of a wire insulated on baked wood, that the electric shock was transmitted instantaneously through the length of 12,276 feet.
Electricity transmitted in large quantities through living vegetables, destroys their vegetable life.
When transmitted, in the same form, through animals, it generally puts an end to animal life; though it is said that there are individuals who are not affected by it. Possibly the reason why some persons are not killed by very large electric shocks is, that their muscular system, or bodily organization, has something peculiar which protects them.
If an electrical circuit be made by means of imperfect conductors, as a slender piece of wood, a wet pack-thread, the discharge will be made silently.
If a small interruption of an electrical circuit be made in water, on making the discharge, a spark willbe seen in the water, which never fails to agitate it and sometimes breaks the vessel in which it is contained.
A strong shock from a battery, sent through a slender piece of metal, instantly makes it red hot. Usually it is melted in whole or in part. If the fusion be perfect it is reduced into globules of different magnitudes. In this experiment it is a little remarkable that the parts of the metal at which the fluid enters and issues, are most likely to be melted.
If the metal be enclosed between pieces of glass, the shock will force the melted metal into the substance of the glass, so that it cannot afterwards be removed, without scraping off part of the glass with it. In this experiment the glasses which enclose metal are commonly broken to pieces.—It is seldom that they resist the force of a strong shock. If the glasses enclosing metal be pressed by a heavy weight, a small shock is often sufficient not only to raise the weight, but to break glasses of considerable thickness. When the pieces of glass are not broken, they are marked by the explosion with the most lively prismatic colours, which lie sometimes irregularly, and sometimes in their prismatic order.
Gun-powder may be fired by a charge from three square feet of coated glass. The powder is to be put into a quill, and then a wire is to be thrust into each end so as nearly to meet, and afterwards these wires are to be made a part of an electrical circuit.—A less charge of electricity will be sufficient if iron filings be mixed with the gun-powder.
When a shock somewhat less than is sufficient to melt a piece of metal is sent through a chain, a black dust, in the form of smoke, is seen to proceed from thechain. This dust is probably some of the metal itself, partly calcined, and by the violence of the explosion forced from it. If the chain be laid upon a piece of paper, glass, or other electric, this, after the explosion, will be found stained with some indelible marks, and often shew evident signs of having been burnt.
What is more remarkable in considering the effects of electricity on metals is, that it often, in a considerable degree, revivifies their calces or oxyds. In making experiments of this kind, the metallic calx or oxyd is to be made a part of an electrical circuit, through which a strong shock is to be sent: when the calx or oxyd will be found in a measure restored to its metallic state: the electric shock having, as it appears, taken away from the oxyd a portion of its oxygen.
The electric shock when passed through the magnetic needle, sometimes destroys its magnetic virtue, and sometimes reverses its poles. It is affirmed that two ships sailing together on the same voyage, were led, from the effect of lightning on their needles, to steer exactly opposite courses, after the storm in which they were exposed to the lightning had subsided. When the charge of ten, eight, or even a less number of square feet of coated glass, is sent through a sewing needle, it will often give it polarity, so that it will traverse when laid upon water. In this experiment it is remarkable that if the needle be lying east and west, that end of it which communicated with the positive coating will point towards the north; but if the needle be struck while lying north and south, that end of it which lay towards the north, will, in any case, point north; and the needle will acquire a stronger virtue in this than in the formercase. But if the needle be placed perpendicular to the horizon, and the electric shock be given to either point of it, the lower extremity will afterwards point north.
The electric explosion taken upon the leaves of certain flowers changes their colour.
If the ball of a thermometer be placed in a strong current of electricity, the mercury or spirit will rise several degrees.
If a thin bottle be exhausted of air by means of an air pump, it will receive a considerable charge of electricity, by applying its bottom to an electrified prime conductor. In performing this experiment the bottle is to be held by the neck or near the mouth, and the electric matter will pass through the vacuum, and along the inner surface of the bottle, to the hand, from that end of it which is nearest to the prime conductor. The luminous appearance exhibited by this experiment is exceedingly beautiful in the dark, especially if the bottle be of any considerable length. It exactly resembles those lights which appear in the northern sky, and which are called streamers or the aurora borealis. If one hand be applied to the part of the bottle which was before presented to the prime conductor, while the other remains at the neck, a shock will be felt, at which instant the natural state of the inner surface is restored by a flash, which is seen pervading the vacuum between the two hands.—The principle on which this experiment depends will be explained hereafter.